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First, as the temperature drops below 32 degrees, ice crystals start to form just beneath the frog’s skin. The normally pliant and slimy amphibian becomes  for lack of a better word  slushy.

Then, if the mercury continues to fall, ice races inward through the frog’s arteries and veins. Its heart and brain stop working, and its eyes freeze to a ghostly white.

“Imagine an ice cube. Paint it green,” and you’ve got the wood frog in winter, said Ken Storey, a professor at Carleton University in Ottawa. The frog is solid to the touch and makes a small thud when dropped.

But it is not dead. When a thaw comes, the frog is able to melt back into its normal state over a period of several hours, restart its heart and hop away, unscathed.

This amazing process of reanimation is being examined by scientists who hope that learning the frog’s secrets might yield clues for improving human medicine, including better preservation of organs on their way to transplant patients.

“Here’s an amphibian that has solved the problem of cryo-preserving its organs  all of them, simultaneously,” said Jon Costanzo, a professor of zoology at Miami University in Ohio. “And we haven’t been able to do that with one [human organ].”

The wood frog, a 2-inch-long creature with a call like a quack, lives in woods from Georgia to Alaska. Other species  the spring peeper and the gray tree frog, as well as a few kinds of caterpillars and the babies of the painted turtle  can freeze, but lose the ability as they age.

Winter survival is basic

Scientists say these animals’ freezing abilities are just an extreme reaction to a problem that all cold-climate animals face: periodic blasts of winter chill. Human retirees head to Florida, Chesapeake Bay crabs bury themselves in the mud, and most frog species hide out deep underground or underwater.

But not the wood frogs. Instead, buried just a few inches under dirt and leaves, they welcome the chill. When the soil starts freezing, so do the frogs.

The result is something like the frozen gray tree frog that professor Jack Layne held in his hand recently in a lab at Slippery Rock University in Pennsylvania.

Instead of its normal grayish-green, the frog had turned almost purple, its limbs and head stuck in contorted positions. It looked for all the world like a practical joke: an ice cube made to resemble a frog.

The frogs can survive this process, in which as much as 65 percent of their body water freezes, because their cells are protected by a kind of natural antifreeze.

Scientists say that, before winter comes, the frogs eat ravenously, storing a starch in their livers. A freeze triggers their bodies to convert the starch into other compounds, most often glucose, or blood sugar. The frogs become, in essence, diabetic.

The glucose lowers the freezing temperature of water inside the frogs’ cells, and because of this, the cells stay liquid, even as ice fills the space around them. This is crucial: If the water inside the cells froze, scientists say, the jagged ice crystals would destroy everything inside, killing the frog.

It’s very hard to find frogs frozen like this in the wild because they’re hidden underground. At the Patuxent Research Refuge, a 12,750-acre forest near Laurel, Md., wildlife biologist Robin Jung, of the U.S. Geological Survey, said she occasionally gets lucky and finds wood frogs hunkered down for winter.

“Just like”  she stiffened as if she’d been shot with a supervillain’s ice ray  “freezing.”

Cold snaps in most of the U.S. usually aren’t long enough to keep the frogs frozen for more than a few days. But wood frogs in Canada and Alaska can freeze for months, scientists said.

New transplant methods?

Medical researchers say they hope to copy these long-term freezing abilities to add hours or even days to the time that human organs can be preserved.

Now, after organs are removed from a donor, they are packed in a special solution and kept on ice. But they can’t be frozen because of the damage that ice crystals would do to the cells. Without freezing, the shelf life of these organs can be as much as 48 hours for a kidney and as little as four hours for a heart.

If organs could be preserved longer, it would allow more time to locate organ recipients and set up the transplant operations, said Jimmy Light, head of transplantation at Washington Hospital Center in Washington, D.C.

“It would allow you to have a more prepared patient,” Light said. “Now, it’s kind of like a fire drill. The bell rings, the clock ticks and you’ve got to get going.”

In one experiment, University of California professor Boris Rubinsky removed a rat’s liver and filled it with glycerol, hoping the chemical would act as glucose does in wood frogs. The experiment worked: The liver was frozen, then thawed and transplanted successfully into another rat, Rubinsky said.

Other researchers have turned to Arctic fish, which manufacture special chemicals to keep from freezing even as the water around them falls below 32 degrees.

Using fish proteins made in a lab, scientists have preserved a pig’s heart at subfreezing temperatures for 24 hours, then transplanted it into another pig.

Scientists say they don’t see any immediate potential for putting an entire human body in a science fiction-style deep freeze; the frogs, after all, don’t stay frozen forever.

But just freezing and thawing one human organ would be a major breakthrough.

“If we can translate that into a human heart, then we’ll do very well,” Rubinsky said.

Now, even as researchers try to copy the frog’s techniques, the freezing amphibians still haven’t given up all their secrets.

Their ability to thaw puzzles scientists, who are trying to crack the process and pinpoint the trigger that restarts the frog’s heart.